Electrochemical capacitors offer significant advantages compared to conventional storage media, such as batteries and capacitors, provide significantly higher energy densities than conventional capacitors, and exhibit higher power and longer cycle life than batteries. Electrochemical capacitors can be separated into two general categories: electrical double layer capacitors (EDLCs) and pseudocapacitors. EDLCs store electrostatic charge at the interface between the electrode and electrolyte, where the charge accumulates on the electrode surface. The most important attributes of an EDLC electrode are high surface area and high porosity, as the amount of charge accumulation is related to exposed surface area.
Recent advances in carbon materials such as carbon nanotubes, two-dimensional one atom thick carbon sheets, and activated carbon (AC) have led to their use as the active material in EDLCs. Two-dimensional one atom thick carbon sheets are one of the most attractive materials for such applications, owing to their remarkably high surface area, excellent electrical and thermal conductivity, electrochemical stability, and mechanical properties. While carbon-based EDLCs can provide a theoretical capacitance up to 550 Farads per gram, this falls short for many practical applications, particularly when compared to electrochemical batteries. Pseudocapacitors, which are based on redox reactions of the electrode material, can have up to 10 times higher capacitance than EDLCs, yet their wide-spread applications have been limited due to lower power density and poor cycling stability.
In pseudocapacitors, only surface and near-surface sites can contribute to charge storage via redox reactions, where the electrode materials are commonly used metal oxides or conducting polymers. Among the metal oxides, ruthenium oxide (RuO2) has been widely studied as a material for pseudocapacitor applications due to its remarkably high specific capacitance (1300-2200 Farads per gram), highly reversible charge-discharge features, wide potential window, and high electrical conductivity (105 siemens per centimeter). For practical applications of RuO2 as a pseudocapacitor electrode, power density and cycle life must be improved.